In the case of known injection pumps such as that described in EP-A-0 027 441 having a camshaft drive, a linear roller tappet is provided between the piston of the pump and an actuating cam. The roller tappet is guided by means of a guide bushing and works against a compression spring, so that is maintains constant contact with the actuating cam. The pump piston, axially aligned with the guide bushing, is guided linearly in a pump cylinder. The linear guidance of the roller tappet is intended to avoid most of the lateral pressures on the pump piston. However, in practice, there is usually a certain amount of center offset between the linear guidance of the pump piston and that of the roller tappet. In addition, it is not possible to avoid slight lateral forces from acting on the pump piston, if for no other reason than that the linear guidance of the roller tappet also unavoidably has free motion.
In the case of the known injection pumps of the aforementioned type, it is also a disadvantage that they have relatively large external dimensions. Accordingly, they cannot be used for all installations, particularly where available space is a factor.
Furthermore, an injection pump is known from U.S. Pat. No. 1,451,228, in which the outer end of the piston of the pump is broadened by means of a screw-on head, on which an auxiliary roller acts. The auxiliary roller is mounted at the end of a rocker arm. Thus, the auxiliary roller makes reliable contact and the piston of the pump is held against it with a compression spring.
With such an injection pump having no separate roller tappet with linear guidance, appreciable transverse forces act on the piston of the pump, which therefore requires a particularly long cylinder guidance.
Also known is a driving mechanism for an injection pump, CH-A-348 289 in which a 3-arm rocker arm produces a positive connection between the camshaft and the piston of the pump. In the case of such an embodiment, for which the rocker arm works without spring loading, the interacting parts must be produced with the highest precision in the region of their contacting areas. As a result, manufacturing costs are particularly high. In addition, conventional types or engine construction do not permit the injection pump to be disposed directly on the camshaft.